This invention relates to ion implantation and, more particularly, to methods and apparatus for improving ion implanatation dose accuracy by reducing measurement errors due to pressure variations in the implant chamber.
Ion implantation has become a standard technique for introducing impurity dopants into semiconductor wafers. A beam of ions is generated in a source and is directed with varying degrees of acceleration toward a target wafer. The ions implanted into the semiconductor material form the various elements of an integrated circuit. Ion Implantation systems typically include an ion source, ion optics for removing undesired ion species and for focusing the beam, means for deflecting the beam over the target area, and an end station for mounting and exchanging wafers. The entire region between the ion source and the semiconductor wafer is maintained at high vacuum to prevent dispersion of the ion beam by collisions with gas molecules.
In commercial ion implantation systems, wafers are introduced into a vacuum ion implantation chamber through an isolation lock, are implanted and then are removed through the isolation lock. In some systems, the wafers remain in the isolation lock and are implanted through an open gate valve. Prior to implantation, the vacuum chamber is maintained at a prescribed baseline pressure level by a vacuum pumping system. When a wafer is introduced into the chamber through the isolation lock, a substantial increase in pressure occurs due to gas introduced with the wafer and outgassing of the wafer and lock surfaces. When the ion beam is applied to the wafer, another pressure increase occurs, due in part to the presence of the ion beam in the chamber, and in part to particles dislodged from the wafer by impact of the ion beam. The gas responsible for the pressure increase, or pressure burst, is removed by the vacuum pumping system, so as to reduce the pressure at a rate determined by several factors described hereinafter. In order to achieve high throughput with the ion implantation system, it is impractical to wait until the pressure has returned to its baseline level. Typically, ion implantation can be performed at a pressure which is an order of magnitude above the baseline pressure. Thus, implantation is begun shortly after the wafers are introduced into the chamber, and, as implantation proceeds, the chamber pressure is gradually reduced.
In the fabrication of microminiature integrated circuits, it is important to implant precisely measured quantities of impurity dopants to achieve predictable device performance. Ion implanters customarily utilize a Faraday charge collection system to measure the ion dosage. In such a system, the wafer is positioned in a Faraday cage which detects the charged particles in the ion beam. The measurement is integrated over time to obtain a measurement of total ion dosage applied to the wafer.
However, the Faraday system accuracy is sensitive to pressure. The residual gas in the vacuum chamber produces errors in the measured dose due to collisions between ions in the beam and residual gas molecules outside the Faraday system. When these collisions occur, some of the ions in the beam are neutralized. Since the Faraday system registers dopant atoms only if they carry an electrical charge, the Faraday system is not able to measure the neutralized portion of the ion beam, and a dose error is introduced. The magnitude of the error depends on the number of neutralizing collisions and, hence, upon the chamber pressure.
Since ion implantation is usually performed during vacuum pumping of gas introduced with the target wafers, the pressure is variable and determination of resulting dose errors is difficult. Several factors cause the pressure level as a function of time during the implant to be unpredictable. Some of these factors are as follows:
(1) The pressure varies with ion beam current.
(2) The pressure at any instant depends on the rate of vacuum pumping, which can vary for a number of reasons.
(3) Undesired leakage into the vacuum chamber causes variations in the pressure versus time curve.
(4) Variations in wafer outgassing, such as water vapor, and particularly outgassing by photoresist mask layers applied to the wafers.
(5) The time of the implant affects the final pressure reached.
(6) Chamber contamination can cause variations in pressure.
The pressure during implantation will vary depending on the above factors, thereby causing an error between the actual dose and the measured dose. In the past, Faraday system designs have been proposed which exhibit reduced sensitivity to chamber pressure. However, it is desired to improve dose accuracy regardless of Faraday system configuration.
It is a general object of the present invention to provide novel ion implantation apparatus and methods.
It is another object of the present invention to provide methods and apparatus for improving the accuracy of ion implanted impurity dosage.
It is still another object of the present invention to provide methods and apparatus for reducing pressure variations during ion implantation.